Cluster exciton strategy enables new sensing applications of room temperature phosphorescence

On July 29, 2022, the team of academicians from the University of Chinese, Shenzhen, Hong Kong, published a research report entitled “Highly efficient and persistent room temperature phosphorescence from cluster exciton enables ultrasensitive off-on VOC sensing” on Matt.

This result uses pure organic host-object strategy to achieve the most simple complex cyclic aromatic hydrocarbon-naphthalene high efficiency and ultra-long room temperature phosphores (RTP), and through experimental and theoretical calculations, it is revealed that the cluster exciton transition state between naphthalene and the host molecule 1,4-dichlorobenzene is an important contribution to promoting interfacial crossing (ISC) and RTP efficiency. The authors demonstrate that naphthalene/1,4-dichlorobenzene host-guest RTP can be used as a powerful spectroscopic tool for detecting naphthalene vapor, with broad application prospects.

The corresponding author of the paper is Academician Tang Benzhong of the University of Chinese, Hong Kong, Shenzhen; Researcher Xuepeng Zhang, Dr. Junkai Liu and Associate Researcher Biao Chen are the co-first authors of the paper.

Naphthalene, the main ingredient in the first generation of mothballs, is easy to sublimate and is still widely used worldwide as a household insect repellent and public toilet deodorant (especially in developing countries), while naphthalene is also a common industrial raw material and an important component of coal tar. Naphthalene vapor is extremely easy to be inhaled by the human body, has strong carcinogenicity, high toxicity to humans, and is flammable and explosive, and has been banned by WHO from being used in mothball products, but there are still many merchants that label naphthalene mothballs as natural mothballs for sale, because naphthalene and natural camphor (a fatty ketone) have a similar appearance and odor, and it is often difficult for law enforcement and ordinary consumers to distinguish. Therefore, there is an urgent need to develop tools that can specifically detect naphthalene molecules.

Long afterglow is a physical phenomenon that can continue to emit light after the external excitation light stops, with high recognition and intuitiveness, the luminous signal is easy to be detected by the naked eye or instruments and can exclude the interference of the excitation light source, which is very suitable as an analytical tool. The most mature long afterglow material is an inorganic doped material, the luminescence principle is that the electron is excited→ captured by the lattice defect→ slowly recombined under thermal energy, although the phenomenon is common, but inorganic afterglow materials often require high processing temperatures, high energy consumption and limited applicable scenarios. Organic afterglow materials have the advantages of flexible and easy to process, good biocompatibility, easy chemical modification, etc., can be widely used in chemical sensing, anti-counterfeiting encryption, optoelectronic devices, biological imaging and other fields, in recent years have received widespread attention, in the organic system, the most common strategy to achieve long afterglow is to use the three-linear exciton to the ground state of the radiation transition (that is, room temperature phosphorescence, RTP). However, pure organic molecules induced without precious metals (platinum, iridium, ruthenium, etc.) often have low ISC rates, which is difficult to efficiently produce trilinear excitons, which becomes a bottleneck in the development of organic long afterglow materials with application value; Especially in pure hydrocarbons that do not contain heteroatoms (O, N, P, Br, I, etc.), spin-orbit coupling is weak and high-efficiency RTP is very rare (Figure 1).

Figure 1: Schematic diagram of the electronic structure and transition of A naphthalene; Schematic diagram of the aggregation quenching effect of B naphthalene; Schematic diagram of C-cluster exciton promoting naphthalene RTP; D Ordinary rigid matrix; E can form cluster excitons with naphthalene and produce a matrix with highly efficient RTP

In this work, Academician Tang Benzhong’s team based on the principle of cluster exciton design (Nat. Commun., 2019, 10, 5161), dispersing naphthalene (NL) as a guest molecule in a very small proportion (e.g., mass fraction one percent or one thousandth) in rigid subject matrices such as 1,4-dichlorobenzene (DCB), 1,2,4,5-tetrachlorobenzene or 1,4-dibromobenzene inhibits the aggregation quenching effect and non-radiative transition of naphthalene molecules on the one hand, and forms excited out-of-bound cluster excitons with the main body on the other hand, promoting ISC, The trilinear excited state of the cluster exciton eventually relaxes to the local trilinear excited state of the naphthalene molecule to emit efficient and ultra-long-life room temperature phosphorescence (Figures 1c and 2).

In particular, in the naphthalene/1,4-dichlorobenzene host-guest combination, the RTP efficiency of naphthalene exceeds 20%, while the green afterglow is more than 10 s (life > 0.7 s), achieving the optimal RTP synthesis performance of heteroatomic thick cyclic aromatic hydrocarbons.

Because there is a large degree of overlap between the excitation spectrum of naphthalene and the excitation spectrum of the subject, the authors speculate that the subject and the object can be in the excited state at the same time after being excited by ultraviolet light, rather than the traditional energy transfer process. The authors verified the charge separation and delocalization of the charge separation and delocalization properties of naphthalene and 1,4-dichlorobenzene and 1,4-dibromobenzene in the excited state through detailed first-principles calculations (completed by Dr. Junkai Liu), respectively, and the cluster exciton transition state increased the COUPLING constant of isC channels and spin orbits, greatly improving RTP efficiency (Figure 3).

Figure 2: Photophysical properties of naphthalene in different matrices

Figure 3: Theoretical calculation of DFT for naphthalene/1,4-dichlorobenzene cluster excitons

Finally, the authors demonstrated that by placing naphthalene mothballs in a closed vessel (quartz tube or PMMA device) together with 1,4-dichlorobenzene, the 1,4-dichlorobenzene solids that do not emit visible light can produce a green afterglow with a peak of naphthalene characteristics that is visible to the naked eye within a few minutes, because the naphthalene molecules rapidly sublimate into the solid matrix of 1,4-dichlorobenzene, forming a host-guest structure, so that cluster excitons and green long-life afterglow can be produced after ultraviolet light excitation.

Figure 4: RTP-lit and specific sensing of 1,4-dichlorobenzene p-naphthalene vapor

This study not only achieves the ultra-high RTP performance of aromatic rings without heteroatoms through the cluster exciton strategy, but also elaborates the principle of tuft excitons promoting RTP through theoretical research. At the same time, the efficient RTP phenomenon of naphthalene/1,4-dichlorobenzene host-guest combination in this work can be used as a powerful spectroscopic tool for visual detection of naphthalene vapor, a common air pollutant, which will be of great significance for product quality control, air quality monitoring and environmental protection. (Source: Science Network)

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